Wear frequently determines the life of a component. Optimizations within a tribological system therefore directly increase the life and thus reduce costs for the user. Components of this type are provided with coatings in order to improve the tribological and wear properties. Coatings display, in a manner analogous to massive materials, various properties which can be determined empirically. These include, for example, hardness, wear resistance, and corrosion behavior in various media. In many applications, however, the frictional behavior of coatings opposite a second friction partner plays a particular role. These are, for example, coated piston rods which run in a guide sheath made of steel or cast iron. The behavior of the friction pairing “coating/friction partner” is of predominant importance in, for example, (internal) combustion engines where coated piston rings run in a bushing made of, for example, grey cast iron or AlSi alloys. In such applications, CrN has in particular been found to be particularly useful. Coatings composed of or containing CrN are therefore widely applied by PVD (physical vapor deposition) to piston rings for (internal) combustion engines, piston compressors and similar piston machines, and also to extruder screws and similar components. Such layers allow good running performance with minimal wear and are now widely established in the motor vehicle sector. A disadvantage is, however, a high capital outlay for plant engineering, which is economical only in the case of large numbers and of small components. In the case of larger components or thicker layers, CrN has hitherto not been applied economically by means of PVD. Stresses caused by different coefficients of thermal expansion of substrate and layer material build up in PVD layers also increases with increasing layer thickness. Such stresses lead to crack formation through to detachment of the layer. This results in insufficient wear reserves being present because the layer thickness is too low for many applications in highly stressed friction pairings. Coatings produced by means of PVD have low roughnesses of less than 10 μm, which is very advantageous for friction pairings. Thermal spraying is an alternative to PVD. Thermal spraying powders are used to produce coatings on substrates. Pulverulent particles are here introduced into a combustion flame or plasma flame which is directed at the (usually metallic) substrate which is to be coated. The particles melt completely or partly in the flame, impinge on the substrate, solidify there, and form the coating in the form of solidified “splats”. Coatings produced by thermal spraying can have a layer thickness up to several 100 μm and often consist of one or more usually ceramic and/or metallic component(s). The metallic component is here able to dissipate thermally induced stresses (residual stress) in the layer by plastic flow, while the ceramic hard phase produces the necessary wear resistance of the layer. Thermally sprayed layers also often have porosities which is advantageous for dissipating stresses.
Wear surfaces having tribologically adjusted friction pairings, in particular piston rings and piston rods, are thermally coated in industry with thermal spraying powders based on molybdenum carbide or chromium carbide in combination with metals and alloys such as nickel, molybdenum, nickel-chromium (“thermal spraying”). This makes it possible to produce layers having a thickness of up to a few 100 μm. Such layers and the spraying powders used consist in each case of at least one metallic component (e.g., NiCrBSi alloy, molybdenum) and a hardness carrier which modulates the wear of the piston ring (e.g., chromium carbides and/or molybdenum carbides).
The intrinsic hardness of these hardness carriers must, however, not be too high since the cylinder surface is otherwise cut. For this reason, hard materials having a high intrinsic hardness, e.g., titanium carbide or tungsten carbide, are not used. It is usual to use carbides which have an intrinsic hardness of less than 2000 HV, e.g., Cr carbides and Mo carbides, as hardness carriers. The latter has an intrinsic hardness of 1900 HV (Mo2C). The particle size of these hardness carriers is preferably as small as possible so as to polish, and not cut, the cylinder surface. This also applies to any additional oxides present, e.g., chromium oxide or aluminum oxide.
Thermal spraying powders comprising hardness carriers can be produced in various ways.
Agglomerated and subsequently intrinsically sintered (sintered together in itself) spraying powders are produced by dispersing (disperging) pulverulent hardness carriers together with metallic binder alloys in powder form (for example Ni or Ni-based alloy powders) in a liquid and then carrying out a granulation step by separating off the liquid, for example, by spray drying. This gives particles which consist of an agglomerated mixture of the powders used. These agglomerates have a mechanical strength which is typically unsuitable for modern spraying processes such as HVOF (“High Velocity Oxygen Fuel”) since these require mechanically stable agglomerates because of the high flame velocities. The spray-dried granulate (granules) is subsequently optionally screened (classified/sized) and intrinsically sintered in a subsequent thermal process step to such an extent that the granulate has a mechanical strength which is sufficient for it not to disintegrate (collapse/degrade) during the thermal spraying process, e.g., by means of HVOF. The thermal process step (“sintering”) is usually carried out either under reduced pressure or under a protective gas which avoids oxidation in the vicinity of atmospheric pressure, usually hydrogen, optionally with proportions of argon and/or other noble gases. This gives a powder or a loosely sintered cake which can easily be converted back into powder, in this case, the spraying powder. The powders obtained are similar in size and appearance to the spray-dried granulate. This intrinsically sintered agglomerate will hereinafter be referred to as “sintered agglomerate”. It is therefore customary in industry to speak of “agglomerated/sintered spraying powders” and of “agglomerated/sintered powders”. The typical internal structure of such agglomerated/sintered spraying powders can be seen from Fig. A.1 in DIN EN 1274 (February 2005). The two powder components (hard material and metallic matrix) can clearly be seen. Agglomerated/sintered spraying powders are particularly advantageous since they offer great freedom in the choice of the components (for example, their contents and particle sizes) and can be readily metered in the spraying process because of their good flowability. It is in particular possible to use very fine hardness carriers which in use leads to very smooth wear surfaces, which in turn leads to low coefficients of friction and high operating lives during use of the friction surface. The particle size of the pulverulent hardness carriers is typically below 10 μm. Particularly finely divided carbides are obtained by reacting metallic components with carbon during sintering, as is practiced in the case of Mo- and NiCr-containing spraying powders.
Sintered and subsequently crushed spraying powders (“sintered/crushed spraying powders”) are produced in a manner analogous to agglomerated/sintered spraying powders, with the difference that the powder components are not necessarily mixed wet in dispersion but can be dry mixed and optionally tableted or compacted to form shaped bodies. The subsequent sintering is carried out analogously, but the temperature and/or any precompaction is effected in such a way that compact, solid sintered bodies are obtained and must be converted back into powder form by action of mechanical force. The powders obtained are therefore irregular in shape and characterized by fracture phenomena on the surface. They also typically have no, or barely any, internal porosity as is typical in the case of agglomerated/sintered spraying powders. Fig. A.6 of DIN EN 1274 (February 2005) shows the typical structure of sintered/crushed spraying powders. The starting powders can barely be discerned. These spraying powders display significantly poorer flowability, which is disadvantageous for a constant application rate during thermal spraying, but is often still practicable.
“Cladded” spraying powders are obtained when the pulverulent hardness carrier is coated with the metallic component by means of electrolytic or electroless deposition. For example, the hardness carrier can be dispersed in pulverulent form in a nickel salt solution, whereupon a shell having a thickness of a few μm is deposited on it by means of electrolytic or chemical reduction. However, this process can be carried out only above a particle size of the hardness carrier of about 10 μm since otherwise, due to the small radii of curvature on the surface of the hardness carrier, the nucleation energies required for fresh formation of the metallic phase increase too greatly and a shell is no longer obtained. The layers obtained after thermal spraying therefore contain relatively coarse hard material particles and thus hardness carriers projecting from the layer surface, which is disadvantageous for a very smooth wear surface. Fig. A.2 of EN 1274 (February 2005) shows the typical shape of a metal-cladded hard material.
A further embodiment of spraying powders composed of a plurality of different powders are “blends”. These are a simple mixture of powders which is then used for coating. However, in the case of modern coating processes, such as the HVOF process, demixing (segregation) of the powder components usually occurs as a result of the high flow velocity and the turbulences, and the composition of the layer therefore no longer corresponds to the composition of the blend.
Hardness carriers which are of particular interest for friction coatings are nitrides. They generally have lower intrinsic hardnesses than the corresponding carbides or even borides. TiN thus has a hardness of 2450 kg/mm2 (for comparison: TiC 3200 kg/mm2). For example, chromium carbides have intrinsic hardnesses in the range from 1880 kg/mm2 (Cr7C3) and 1663 kg/mm2 (Cr23C6), whereas Cr2N has a hardness of 1591 kg/mm2, and CrN a hardness of only 1093 kg/mm2. It is clear from this why pure CrN has become established as coating material for piston rings. While Cr2N has an intrinsic hardness of the same order of magnitude as chromium carbides, and is thus tribologically suitable for friction pairings, CrN has a lower intrinsic hardness. The far higher hardnesses measured for PVD coatings are due to residual stresses and the particular substructure of the coating and must not be compared with the hardnesses determined on crystallites (“intrinsic hardnesses”).
Chromium nitrides also have excellent resistance to frictional wear and, due to their pronounced chemical inertness, are insensitive to microwelding phenomena which must be avoided in many uses because of the resulting adhesion wear.
It would therefore be desirable to have agglomerated/sintered spraying powders having a metallic component such as nickel and containing chromium nitrides as hardness carriers. These would make it possible to produce thicker layers which would have sufficient wear reserves.
Agglomerated/sintered spraying powders or sintered/crushed spraying powders (in the present disclosure described collectively as “sintered spraying powders”), in particular ones containing CrN, have hitherto not been described. The reason therefor is that decomposition of the CrN into Cr2N, from Cr2N to metallic chromium and, depending on the presence of carbon during sintering, also a further reaction to form Cr carbides, whose intrinsic hardnesses are all higher, occurs during sintering of chromium nitride-containing granulates or powder mixtures. Owing to the rapidity of the spraying process and the splitting-off of the nitrogen being slower compared to heat transport due to diffusive transport, it can be assumed that sintered spraying powders could also produce chromium nitride-containing coatings if sintered spraying powders of this type could be produced.
Owing to the high melting points in the production of atomized spraying powders, the nitrogen content necessary for formation of significant contents of chromium nitrides cannot be obtained in the melt since the solubility of nitrogen therein is too low.
A further possible way of producing chromium nitride-containing coatings is the use of powder mixtures (“blends”), for example, mixtures of Ni or NiCr powder with chromium nitrides and optionally other hardness carriers. A disadvantage is, however, that comparatively coarse hardness carriers must be used in order that the oxidation thereof is sufficiently slow during thermal spraying and sufficient kinetic energy is present on impingement. Typical particle sizes for hardness carriers and matrix metal are in this case from 10 to 100 μm. Layers produced in this way accordingly have high roughnesses and a poor distribution of hardness carriers in the metallic matrix. Blends are therefore not alternatives.
DE 10 2008 056 720 B3 describes the production of a sprayed layer, which serves as sliding element in an (internal) combustion engine, from chromium nitride-containing spraying powders, whose production process is not disclosed. The sliding layer has a nominal composition of from 10 to 30% of Ni, from 0.1 to 5% of carbon, from 10 to 20% of nitrogen, and from 40 to 79.9% of chromium. The spraying powder which is described in the working example and whose production method is unknown had a nominal composition of 60% of CrN, 10% of Cr3C2, 25% of Ni, and 5% of Cr. The homogeneous distribution of the carbides (i.e., the 10% of Cr3C2 contained in the spraying powder) in the sprayed layer is described. The size and distribution of the CrN is likewise not disclosed. The CrN used led, in the elemental analysis, to only 11% of nitrogen instead of the theoretically to be expected 12.72%. It can therefore be deduced that the chromium nitride component described as “CrN” cannot be pure CrN since otherwise a nitrogen content of 12.7% would be expected in the elemental analysis. It can be calculated from the indicated 11% of nitrogen that the chromium nitride component present to an extent of 60% in the spraying powder consisted of only 41% of CrN containing 21.2% of N and of 19% of Cr2N containing 12.1% of N, i.e., it consisted of 68.3% of CrN and 31.7% of Cr2N. According to the disclosure, the wear properties of the CrN PVD coating were therefore presumably not achieved (Table 1 of DE 10 2008 056 720 B3). The powder disclosed also contains chromium carbides, which can be seen from the material system disclosed, the structural micrographs of the sprayed layer (“homogeneously distributed carbides”) and the elemental analysis. Owing to the high intrinsic hardness of the chromium carbides, the chromium nitride-based sliding coating cannot display its full potential and is not comparable in terms of performance with the CrN coating produced by means of PVD.